Download Methods in Cognitive Neuroscience I

Cognitive Neuroscience
Exploring Brain/Behavior
relations
Neuroscience
Psychology
Cognitive
Neuroscience
Computational Sciences /
Artificial intelligence
Franz Joseph Gall & J. C. Spurzheim –
localization of different psychological
functions to different regions of the
cerebral cortex (late 1700’s – early
1800’s)
Franz Joseph Gall & J. C. Spurzheim –
localization of different psychological
functions to different regions of the
cerebral cortex (late 1700’s – early
1800’s)
- phrenology
The brain hypothesis: functional
specialization or distribution?
1810
Brain Hypothesis
• Mass action (Lashley, 1930s) and aggregate field theories
Flourens (1794-1867)
Older Methods
• Cognitive Psychology
– Behavior is the basic unit of study
– Phenomena must be well characterized
Cognitive Psychology
• Has benefited as a science by the
development of a circumscribed set of
methods and techniques
• Basic methods have yielded a number of
phenomena in need of explanation
Measurement of Human Performance in
Information Processing Tasks
Basic Units of measurement:
Reaction time
Accuracy
• Much work has been done to establish the
validity and reliability of these measurements
W. W. Norton
The Posner Task
Results demonstrate that multiple representations are
activated by a single stimulus
The Word Superiority Effect
• A target letter can be identified more quickly when it is
imbedded in a word than when it appears among a
random letter string
The Sternberg Task
• RT increases
monotonically with
increasing memory setsize
• Similar RT slopes for
both “yes” and “no”
responses
Implications of Sternberg
Task Results
• Memory retrieval is a serial comparison
process between items in memory and
those in the world
• Each comparison takes a fixed amount of
time
• Mental operations can be quantified in
terms of the amount of time they take
The Stroop
Effect
• Subjects take longer to
name a color word (e.g.,
red) when it is printed in
a color that does not
match the word
Implications of the Stroop
Effect
• Multiple representations
• “Privileged access” of some
representations over others
Older Methods
• Neuropsychology
The study of cognitive deficits following
brain damage
Older Methods
• Neuropsychology
The “lesion method”
The role of a missing brain region may
be inferred from what the patient cannot
do after it is removed
The Lesion Method
• Laid the empirical foundation for
modern cognitive neuroscience
– Broca: Left hemisphere language dominance
The Lesion Method
• Logic is based on a localizationist
perspective
• Does not take into account the adaptive
“parallel” nature of brain function
Neuropsychology Methods
• Basic question: Is brain region X
important for Task A?
Single Dissociation
Patients
w/lesion to region X
Task A
Task B
(recency memory)
Familiarity memory
92%
70%
(temporal lobe)
Controls
90%
94%
1 Patient group, 1 Control group
Two tasks,
Difference between groups only occurs in Task B
Inference Problems with
Single Dissociations
• Both tasks assumed to be equally
sensitive to group differences
• Single dissociation may result from
general effects of trauma, not specific
effect of lesion
Double Dissociation
Patients
w/lesion to region X
Task A
Task B
recency memory
Familiarity memory
92%
70%
64%
89%
90%
94%
(temporal lobe)
Patients
w/lesion to region Y
(frontal lobe)
Controls
2 Patient groups
1 Control group
Patient groups differ on task affected, control group unaffected
Representations in CNS
Different functions are represented
in different brain regions.
Gross Anatomy: Anatomical division
4 lobes
Functional Divisions of the Cerebral Cortex
1. SENSORY CORTECES
a. Motor Areas of the Frontal Lobe
b. Somatosensory Areas of the Parietal Lobe
c. Visual Processing Areas of Occipital Lobe
d. Auditory Processing Areas of the Temporal
Lobe
a. Motor Areas of the Frontal Lobe
b. Somatosensory Areas of the Parietal Lobe
c. Visual Processing Areas of Occipital Lobe
d. Auditory Processing Areas of the Temporal Lobe
Auditory
Cortex
The Emergence of
Cognitive Neuroscience
• Fueled by the development of powerful
new imaging instruments and techniques
• Formulation of questions based on
discoveries with older, more established
methods
• Relies critically on converging operations
between new methods and older
established methods
Cognitive Neuroscience
Methods
The major methods
•
•
•
•
Single-unit recording
Lesion studies
Transcranial magnetic stimulation (TMS)
Neurosurgery-related methods
– Direct cortical stimulation
– Split-brain
– WADA
• Functional imaging
– Electromagnetic: EEG, MEG
– Hemodynamic: PET, fMRI
Single unit recording
• Used extensively in animal studies
• A microelectrode is inserted into brain tissue
and recordings of action potentials can be made
from nearby neurons, ideally a single neuron.
– Recordings are typically extracellular
• The animal can then be presented with various
sensory stimuli, or trained to perform some
task, and the effects on neural activity can be
monitored
• Advantages: great spatial and temporal
resolution
• Disadvantages: sampling only a very small
fraction of a functional neural system
Transcranial Magnetic
Stimulation
• A method for producing temporary focal
brain “lesion” (disruption), via
stimulation with a strong magnetic field.
• With milder fields, can produce
“excitation” or facilitation effects.
Transcranial Magnetic Stimulation
• Coil placed over target brain region
• Cognitive failures recorded
TMS - Virtual lesions
The magnetic fields used in TMS are produced by
passing current through a hand-held coil,
whose shape determines the properties and size
of the field. The coil is driven by a machine
which switches the large current necessary in a
very precise and controlled way, at rates up to
50 cycles per second in rTMS. Small induced
currents can then make brain areas below the
coil more or less active, depending on the
settings used.
TMS
Error rates in letter identification
as a function of the coil location
over the occipital pole (Corthout
et al., 1999)
Proportion error (p)
• Previous studies have
demonstrated
suppression of visual
perception with TMS
over the occipital
cortex (letter
detection, trigram
recognition)
1
0.8
0.6
0.4
0.2
0
Control
TMS
Neurosurgery Methods
• Direct cortical stimulation
– Delivery of a small electric current directly on the
cortical surface
– Causes temporary disruption or facilitation of
function in cortex being stimulated
– Used clinically to map function, so that critical
regions can be avoided during tissue resection
– Can be done intra-operatively, or more commonly
now, via chronically implanted electrode grids
Neurosurgery methods (con’t)
• Split-brain
– Sectioning of corpus callosum as a treatment
for medically intractable epilepsy
– Can study the separate contributions of the
left and right hemispheres to various
abilities/tasks
W. W. Norton
Neurosurgery methods (con’t)
• WADA procedure
– Injection of sodium amytal (a barbituate),
into one and then the other carotid artery
temporarily (5-10min) puts half the brain to
sleep allowing neurologists to assess function
in the awake hemisphere
W. W. Norton
Neurosurgery methods (con’t)
• General considerations
– Advantages: better experimental control in
some situations, e.g., temporary lesions can
be very focal and reversible
– Disadvantages: all subjects in these subjects
are undergoing these procedures because
they have a neurological disorder, therefore
it is not clear how generalizable the results
are.
Functional imaging
• Electroencephalography (EEG)
– Scalp electrodes measure the summed
electrical activity of large populations of
synchronously active neurons
– Can look at the changes in this signal as a
function of mental activity
• Changes in synchrony of different populations of
neurons
• Changes in morphology of EEG signals that are
time-locked to an event (e.g., a perceptual
stimulus), this is called event-related potentials
(ERPs)
W. W. Norton
Functional imaging
• Magnetoencephalography (MEG)
– Measures magnetic fields associated with
large populations of synchronously active
neurons
– Can measure synchrony or event-related
changes in the signal like EEG
Functional imaging
• Electromagnetic techniques -- general
considerations
– Very good temporal resolution (milliseconds)
– Generally poor spatial resolution (roughly
on the order of the size of a cerebral lobe)
– For simple sensory or motor events
resolution can be better (closer to 1 cm),
particularly for MEG
Functional imaging
• Positron emission tomography (PET)
– Involves injection of radio-label oxygen or
glucose into the blood stream, and measures
the location in the brain that this material
accumulates
– Good spatial resolution (~1cm)
– Poor temporal resolution (~1 min at best)
Positron Emission Tomography
Capitalizes on blood-flow or
“hemodynamic” properties of
brain
• Subjects injected with radioactive isotop
• Measures local changes in blood flow that are linked to neural
activity
• Neural activity => increased metabolic demand => local increase
in blood flow the active region
Example of a PET Experiment
• Petersen, Fox, Posner, Mintun & Raichle
(1988) PET using radioactive O2-tracer
• Wanted to understand brain bases of word
processing
• Subjects performed several tasks
– Looking at words
– Listening to words
– Saying words aloud
• either read or heard
– Thinking of words
• word association task (apple-orange, sleep-bed)
Example Results
looking at words
speaking words
listening to words
thinking about words
Functional imaging
• Functional magnetic resonance imaging
(fMRI)
– Like PET, fMRI measures regional changes
in blood flow, but does it very differently
– As blood flow increases, so does the oxygen
concentration in the blood. MRI is sensitive
to these O2 concentration changes
– Excellent spatial resolution (3-6mm),
relatively poor temporal resolution (on the
order of seconds)
Structural MRI
• Takes advantage of the fact that different types
of tissue produce different radio-frequency (RF)
pulses
Functional MRI
• Takes advantage of the fact that neural activity is
followed by blood flow in a highly predictable manner
• Altered blood flow alters RF signal from active brain
regions
Functional MRI
• Permits examination of brain regions
that become active during cognitive
performance
cognitive task
baseline
Facilitates comparison of brain activity in younger and older
fMRI vs. PET
• BOTH: blood flow to brain provides the
signals detected
– when resting neurons become active, blood
flow to them increases
• fMRI - detects changes in oxygen levels, which
rise in nearby blood vessels when they are at
rest
• PET - relies on increased delivery of injected
radioactive water, which diffuses out of the
vessels to reach rest of brain.
Some methods have better spatial
resolution;
Some have better temporal
resolution;
Some have functional
resolution.